Electronic members are used to store large quantities of digital data and other information used in computers and other computerized electronic devices in which large amounts of information need to be recorded and/or retrieved and displayed upon a cathode ray tube or other video display device. In more recent years, forms of digital storage devices have been incorporated within computerized entertainment devices such as those used for presenting video information, pictures as well as digitally stored music. A typical form of storage device employs a rotatable storage medium or "disc", either permanently housed within the device or which is removable and may be removed and replaced by another disc; and a transducer or "head" by means of which the information stored on the disc is interrogated or "read out" and coupled to other devices in the system. Disc drives of this type include magnetic disc drives in which the discs are of magnetic material and in which the information is stored in the form of magnetic flux, whereby the information is read out by magnetic type transducer means, and optical disc drives in which the information is stored in the form of pits or other broader optical discontinuities in the disc material and in which a readout is accomplished by light transducer means, typically a laser diode and photodetector combination. The information is arranged and stored in the "tracks" located on the disc. Those tracks may be in the form of a continuous spiral track or a series of concentric circular tracks.
Information is stored by filling the disc tracks with closely spaced disc continuities; so called "pits" for optical discs, magnetic flux reversals for magnetic discs. In a typical arrangement for an optical disc, there may be approximately 5,000-10,000 of such "pits" in each centimeter of circumferential track length.
The information stored is retrieved by a pick-up transducer, suitably a servo driven head which contains a sensor, either optical or magnetic. The servo positions the head over a selected track and the sensor reads the information previously stored on that track. In additional to reading the information on a particular track, a provision in the system provides rapid switching between one track and another, which may be spaced some distance apart on the disc; an action that is referred to as "seeking" in magnetic disc drives and/or "track jumping" in optical disc parlance. In the operation of a computerized device a "command" is given to procure certain information from within storage. Given the identifying information, the command causes the sensor to seek the track location in which the particular information is stored, following which, the head or sensor retrieves the stored information and returns it to the other electronic circuits within the computer for further processing or for display.
In particular, interactive video used that is often used for games, education, training, and the like, frequently requires a "branching" function; that is, instantaneous switching to a new scenario under user control. The branching is accomplished by interleaving a number of scenarios on the disc track. The user initiates a branch by causing the video disc player to execute "track jumps" in a sequence which selects one of the interleaved scenarios. In order to create a steady video image, the track jumps must be accomplished within the video display's vertical retrace inverval and the number of tracks in each jump must be precisely controlled.
The foregoing retrieval operations require that the head and sensor be moved rapidly in a radial direction with a controlled motion so that the sensor comes to rest over the desired track. Various technologies for accomplished sensor positioning in this arrangement are known and are in wide use. Those existing techniques are, however, either quite elaborate, typically requiring a separate channel for track jumping which is electronically switched in at the beginning of a track jump, followed by a switch out at the conclusion, or is of crude design in that only short track jumps can be executed with acceptable levels of precision. The present invention provides a relatively simple means to permit highly accurate track jumps of any length.
With existing technology, the first approach to head positioning is accomplished in a three-step process. First, the tracking servo loop is opened and an "accelerate signal" is applied to the sensor head. Second, at the mid-point of head travel between the two track positions, the signal applied to the sensor head is changed to a "decelerate" signal. And, at the conclusion of travel, the "decelerate" signal is removed and the tracking servo loop is closed which, ideally, leaves the sensor positioned over the correct disc track. Because there is no closed loop control of the sensor head during a track jump, this approach is believed to be suited only to short jumps.
The existing technology for long "jumps" or "seeks" contains some means for monitoring the movement of the sensor as it travels from its start position to the selected disc track. This uses a closed loop control insuring that the sensor remains on course, so to speak, and arrives at the correct track, even after lengthy travel. Typically, the velocity of the sensor movement is a parameter used for head positioning control. In addition, a technique for sampling position error once for each track crossing is presented in U.S. Pat. No. 4,547,822, granted to Stewart Brown, the present application.
An advantage of the present invention is that it avoids the need for an additional channel and the accompanying electronic hardware as in the case of the prior drives.
A principle object of the invention is to provide an improved control for positioning a moveable disc head transducer. A further object is to provide accurate track jumps under adverse conditions such as video disc with dust, fingerprints, etc., on the optical surface. A further object is to provide a disc head control circuit that accurately positions the head even over long jumps and is less expensive than existing techniques that perform that function. A still additional object is to provide a lowcost disc head position controller capable of accurately positioning the head.